1. Field of the Invention
The present invention pertains to antennas for transmitting and/or receiving electromagnetic radiation. More particularly, the present invention relates to techniques for communicating polarized radiation in broad frequency bands or multiple frequency bands by using antenna assemblies of microstrip construction.
2. Description of Prior Art
The use of antennas for both transmitting and receiving electromagnetic radiation is well known. A variety of approaches to antenna construction is followed, with a particular antenna design selected generally on the basis of several factors. These factors may include, among others, the frequency range in which the antenna is to function, physical size limitations on the antenna, mobility requirements, if any, the radiation pattern desired, and the nature and degree of the polarization of the radiation to be communicated. All of these factors are particularly critical when the antenna in question is mounted for operation on an aircraft or spacecraft. In such case, for example, the antenna must generally be small and lightweight and able to function at a variety of orientations of the craft relative to the ground, and the station with which the craft is communicating. Also, the aerodynamic design features of the antenna may be critical on a craft intended to move through the atmosphere at high speed. Particularly in the case of a spacecraft, located at high altitudes and communicating with a ground level station, polarization effects can reduce signal strength significantly. For example, waves polarized in one direction relative to the plane of incidence of a wave front reflecting at a boundary between atmospheric layers may undergo a phase change, while waves polarized at a different angle may not experiencing a phase change. An interference pattern may then result with a spacecraft experience fluctuating signal strength as the antenna is moved through a pattern of nodes.
My co-pending U.S. Pat. Application Ser. No. 750,796, filed Dec. 15, 1976 discloses an antenna assembly including an antenna element that is an elliptical lamina in shape and is separated from a ground plane by a dielectric lamina. The frequency range within which the antenna assembly is operable for communication of electromagnetic radiation is determined by the size of the antenna element. The polarization of the electromagnetic radiation is determined by the location of the feed point on the antenna element through which the assembly is coupled, with respect to the axes of the elliptical lamina.
U.S. Pat. No. 4,060,810 discloses an antenna assembly including two planar antenna elements, one generally circumscribing the other. Each antenna element is operable within a different frequency range, to provide a two-frequency antenna assembly.
U.S. Pat. No. 3,665,480 discloses the use of multiple antenna elements to reform the radiation pattern of a single signal. U.S. Pat. No. 3,854,140 discloses an antenna array for communicating two beams at different frequencies.
Microstrip antennas, particularly those operating to communicate circularly polarized radiation, have been generally characterized by narrow bandwidth operation. Consequently, such antennas are substantially single-frequency antennas.